1. Field of the Invention
This invention relates generally to an amplitude modulator and, more particularly, is directed to an amplitude modulator which can finely adjust an amplitude modulation degree.
2. Description of the Prior Art
An RF (radio frequency) converter for frequency-converting an input video signal into a television signal for a vacant channel is generally constructed as shown in FIG. 1.
In FIG. 1, reference letter e.sub.V designates an input video signal and a reference letter e.sub.A designates an input audio signal. The input video signal e.sub.V is supplied through an over-modulation preventing limiter 2 to an amplitude (AM) modulator 10 as a video signal e.sub.V ' in which an AM carrier signal fc (video carrier frequency of a vacant channel) is amplitude-modulated by this input video signal e.sub.V ' and then an output signal, which is frequency-converted to a television signal of the vacant channel, is developed at an output ternminal 10A.
By the way, when it is required for the RF converter thus constructed to finely adjust the amplitude modulation degree of the output signal, in the prior art, it is general that the amplitude modulation degree is adjusted finely by adjusting the amplitude of the input video signal e.sub.V. However, when the amplitude modulation degree is adjusted by adjusting the amplitude of the input video signal e.sub.V, there occur the following problems.
Although it is desirable that a difference between a modulation degree at a normal state and a maximum modulation degree determined by the limiter level of the limiter 2 is always constant, if the amplitude of the input video signal e.sub.V is varied, the difference between the modulation degree at the normal state and the maximum modulation degree is also varied. For example, if the amplitude of the input video signal e.sub.V is increased so as to increase the modulation degree at the normal state, the difference between the modulation degree at the normal state and the maximum modulation degree is reduced so that in such case, troubles, such as the deterioration of a white level of the video signal and the like will easily occur.